Thin film transistors (hereinafter referred to as TFTs) have been widely used as pixel switching elements for a display unit such as a liquid crystal display and an EL display. In recent years, pixel array driver circuits have also been increasingly formed on the same substrate by the use of TFTs. Such TFTs have conventionally been prepared on a glass substrate using amorphous or polycrystalline silicon. However, unfortunately, a CVD apparatus used for preparing a TFT using such silicon is very expensive, leading to a significant increase in manufacturing cost associated with producing a larger-area display unit using TFTs. Further, since a process for depositing amorphous or polycrystalline silicon is performed at an extremely high temperature, there are limits to materials which can be used as a substrate; specifically, a lightweight resin substrate or the like cannot be used.
As a means of solving the problem described above, a TFT using a CNT dispersed film as a semiconductor material has been proposed. The CNT dispersed film is generally formed by a wet process such as a coating process. The wet process can inexpensively realize enlargement of the area and requires a low process temperature. Therefore, an advantage of this process is that there are few limits to selecting a material used as a substrate. For this reason, practical utilization of TFTs using a CNT dispersed film is expected. Actually, TFTs using a CNT dispersed film have been extensively reported in recent years, for example, in the following Non-Patent Documents 1 to 4.    Non-Patent Document 1: E. S. Snow, J. P. Novak, P. M. Campbell, and D. Park, Applied Physics Letters, vol. 82, p. 2145, 2003    Non-Patent Document 2: E. Artukovic, M. Kaempgen, D. S. Hecht, S. Roth, and G. Gruener, Nano Letters, vol. 5, p. 757, 2005    Non-Patent Document 3: S. -H. Hur, O. O. Park, and J. A. Rogers, Applied Physics Letters, vol. 86, p. 243502, 2005    Non-Patent Document 4: T. Takenobua, T. Takahashi, T. Kanbara, K. Tsukagoshi, Y. Aoyagi, and Y. Iwasa, Applied Physics Letters, vol. 88, p. 33511, 2006